The invention relates to a method for regenerating a NOx storage catalytic converter for an internal combustion engine.
To further reduce the fuel consumption of spark-ignition internal combustion engines, internal combustion engines with lean combustion are being used with increasing frequency. In the case of spark-ignition internal combustion engines with lean combustion, the air excess is chosen to be great enough to allow the load requirement on the internal combustion engine. When there is a low load requirement, for example when there is a low torque or low or no acceleration, the fuel/air mixture with which the internal combustion engine is operated in stratified-charge mode may have lambda values of 3 and more.
To meet the exhaust emission limit values required, a special exhaust treatment is necessary in the case of such internal combustion engines. NOx storage catalytic converters are used for such purposes. These NOx storage catalytic converters are capable, on account of their coating, of absorbing from the exhaust gas NOx compounds that are produced in a storage phase in lean combustion. During a regeneration phase, the absorbed or stored NOx compounds are converted into harmless compounds by adding a reducing agent. CO, H2, and HC (hydrocarbons) may be used as the reducing agent for lean-operated spark-ignition internal combustion engines. These are generated by briefly operating the internal combustion engine with a rich mixture and are made available to the NOx storage catalytic converter as components of the exhaust gas, whereby the stored NOx compounds in the catalytic converter are broken down.
European Patent Application EP 0 560 991 A1 discloses an absorber catalytic converter system with lean-mix operation, in which the switching over between lean operating phases and stoichiometric or enrichment operating phases takes place at comparatively roughly estimated points in time. In this case, the internal combustion engine is run for as long as possible in lean operation and then switched from a stoichiometric or enrichment operating phase back again to lean operation if it is assumed on the basis of a corresponding estimate that the NOx adsorber has regenerated again.
It is accordingly an object of the invention to provide a method for regenerating an NOx storage catalytic converter in the exhaust gas stream of an internal combustion engine that overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and that determines the triggering point in time for the regeneration phase as accurately as possible.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for regenerating a NOx storage catalytic converter disposed in an exhaust gas stream of an internal combustion engine operating with air excess, at least in certain operating ranges, which converter stores NOx contained in the exhaust gas of the internal combustion engine in a storage phase, and which converter catalytically converts stored NOx in a regeneration phase, including the steps of disposing a NOx sensor downstream of a NOx storage catalytic converter for evaluating a signal of the NOx sensor, detecting the signal of the NOx sensor in the operating range of the overrun fuel cut-off of the engine, forming at least one of an average value and a minimum value from individual measured values of the signal and storing the at least one of the average value and the minimum value as an offset value of the signal, in operating ranges of the engine beyond the overrun fuel cut-off, detecting the signal of the NOx sensor and taking into account the offset value when the signal is processed further, comparing an offset-corrected signal with a threshold value, and emptying the NOx storage catalytic converter by catalytically converting stored NOx through addition of a reducing agent in a regeneration phase in the event that the threshold value is exceeded.
According to the invention, the signal of the NOx sensor is detected in the operating range of the overrun fuel cut-off of the internal combustion engine and an average value is formed from the individual measured values. The average value is stored as an offset value of the sensor signal. In operating ranges of the internal combustion engine beyond the overrun fuel cut-off, the signal of the NOx sensor is detected and the offset value is taken into account when the signal is processed further. The offset-corrected signal is compared with a threshold value. In the event that the threshold value is exceeded, a regeneration phase of the NOx storage catalytic converter is initiated.
According to one development of the method according to the invention, in the operating range of the overrun fuel cut-off, the air mass flowing into an intake tract of the internal combustion engine is summed and the determination of the offset value is enabled only when the air-mass sum exceeds a threshold value. Such determination ensures that the NOx storage catalytic converter is adequately purged with fresh air and that NOx from the last combustion is no longer present in the exhaust gas, which may falsify the adaptation of the offset of the sensor signal.
In accordance with a further mode of the invention, there is provided the step of determining the offset value by an arithmetic and/or sliding averaging of the individual measured values.
In accordance with an added mode of the invention, there is provided the step of fixing the threshold value for initiating the regeneration phase as a function of the air mass stream in the intake tract.
In accordance with a concomitant feature of the invention, there is provided the step of storing the threshold value in a characteristic map of a memory of a control device for controlling the internal combustion engine.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for regenerating a NOx storage catalytic converter for an internal combustion engine, it is nevertheless not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.